This invention concerns improvements in and relating to fluorine-containing polymers, and in particular to terpolymers of tetrafluoroethylene (TFE), hexafluoropropylene (HFP) and vinylidene fluoride (VF.sub.2).
Fluorinated polymers such as polytetrafluoroethylene (PTFE), polyhexafluoropropylene (PHFP), polyvinylidene fluoride (PVF.sub.2) and copolymers of tetrafluoroethylene and hexafluoropropylene (FEP) are known as heat-sealable coatings for polyimide film in U.S. Pat. No. 3,352,714. Numerous other patents, such as U.S. Pat. No. 3,592,714, also disclose FEP as a heat-sealable coating for polyimide film. However, polyimide film coated with PTFE or FEP requires a temperature for heat-sealing which is too high for some uses, such as insulation over tin-coated copper conductors, because the temperature required for heat sealing damages the tin coating, thereby impairing the solderability of such conductors. Coatings which are more stable than PVF.sub.2 are desired, and coatings of PHFP are very tacky.
Other fluorine-containing polymers are known in the art.
U.S. Pat. No. 2,968,649 discloses and claims a terpolymer consisting of 3 to 35% by weight of tetrafluoroethylene units and 97 to 65% by weight of vinylidene fluoride and hexafluoropropene units, in which the vinylidene fluoride and hexafluoropropene units are in a weight ratio in the range of 2.33:1 to 0.667:1.0.
U.S. Pat. No. 3,801,552 discloses a method of producing a fluoroelastomeric terpolymer by polymerization of tetrafluoroethylene, vinylidene fluoride and hexafluoropropene. The composition of the terpolymer falls within the area bounded by the following four mole ratios of tetrafluoroethylene, vinylidene fluoride, and hexafluoropropene: 5:85.7:9.3, 5:59.7:35.3, 30:28:42, and 30:54:16.
U.S. Pat. No. 4,123,603 discloses and claims a terpolymer consisting of 10 to 14% by weight of tetrafluoroethylene, 27 to 31% by weight of hexafluoropropylene and 57-61% by weight of vinylidene fluoride.
U.S. Pat. No. 4,141,874 discloses a copolymer of vinylidene fluoride and at least one other fluorine-containing monomer, said copolymer containing 28 to 92% by mole of the vinylidene fluoride unit and having an intrinsic viscosity of 0.4 to 1.3 at 35.degree. C. in methyl ethyl ketone, said other fluorine-containing monomer being at least one member selected from the group consisting of tetrafluoroethylene, trifluoroethylene, trifluorochloroethylene, trifluoropropene, hexafluoropropene, pentafluoropropene, trifluorobutene, perfluoromethyl perfluorovinyl ether and perfluoroethyl perfluorovinyl ether, which copolymer is prepared by copolymerizing at a first stage vinylidene fluoride and at least one other fluorine-containing monomer in an aqueous medium in the presence of a water-soluble radical polymerization initiator to give a first stage-copolymer having an intrinsic viscosity of 0.01 to 3.0 at 35.degree. C. in methyl ethyl ketone, and further copolymerizing at a second stage vinylidene fluoride and at least one other fluorine-containing monomer in an aqueous medium in the presence of said first stage-copolymer and an oil-soluble radical polymerization initiator to give second stage-copolymer, said elastomeric copolymer comprising 1 to 80% by weight of said first stage-copolymer and 20 to 99% by weight of said second stage copolymer.
Belgian Pat. No. 844,965 discloses a terpolymer consisting of 50 to 85% by weight of tetrafluoroethylene, 5 to 40% by weight of hexafluoropropylene and 10 to 45% by weight of vinylidene fluoride.
Japanese Patent Publication No. 48-18957 discloses a process for preparing fluorine-containing terpolymers containing 10-30% by weight of tetrafluoroethylene and 90-70% by weight of vinylidene fluoride and hexafluoropropene having weight ratio of vinylidene fluoride to hexafluoropropene in the range 1.6:1 to 4:1.
In addition, a fluorine-containing terpolymer has been marketed in the United States by Daikin Kogyo Co., Ltd., Osaka, Japan, in which the composition by weight of tetrafluoroethylene, vinylidene fluoride and hexafluoropropene was about 14:53:33.
However, none of the known fluorine-containing polymers provides a coating for polyimide or other heat-stable polymer films which is capable of being formulated to provide all of the desired characteristics. These characteristics include thermal stability, good heat-sealing characteristics, and antiblocking properties. The heat-seal should be strong not only initially, but should remain strong after extended aging at high temperature, after exposure to boiling water, and after exposure to certain detergents and solvents. The terpolymer should be such that the coated film is antiblocking for coated-side-to-uncoated-side in the case of one-side-coated film, and preferably, is also antiblocking for coated-side-to-coated-side in the case of two-side-coated film.
It is accordingly an object of the present invention to provide terpolymers of TFE, HFP and VF.sub.2 which are capable of being formulated to provide the desired thermal stability, heat-sealing and antiblocking characteristics in a heat-sealable coating for polyimide and other heat-stable films.